1. Field of the Invention
The invention relates to a process for producing vacuole-free, heat-protected, round monofilaments composed of polyamide for industrial fabrics.
2. Prior Art
The production of monofilaments for reinforcing rubber articles, especially for tires, is known. U.S. Pat. No. 5,262,099 discloses a process for producing monofilaments by extrusion into water, which have oval-shaped diameters. Although linear densities of up to 12,000 denier (about 13,000 dtex), corresponding to diameters of about 1.3 mm, are mentioned, the maximum linear density which is exemplified is 6000 denier (about 6700 dtex), corresponding to diameters of about 0.85 mm. The production of monofils&gt;0.8 mm in diameter presents cooling problems with extrusion into water, which become apparent through the formation of vacuoles in the final monofilament. These monofilaments are unsuitable for producing fabrics for industrial purposes, since end breakages are a frequent occurrence and yarn properties are not constant.
EP-A-0 230 228 discloses producing monofils having a round diameter from a mixture of polyamide 66 and polyamide 6 for spiral wires for the paper industry. However, it was found that the total proportion of PA 6 must not exceed 15% by weight. The known process produces polyamide monofils 0.3 to 0.7 mm in diameter. These known diameters are too small to give rise to vacuoles in the monofilament.
There has long been a need to use coarser monofilaments for tire cord. By coarse monofilaments are meant monofilaments having a linear density of more than 8000 dtex, corresponding to a filament diameter of more than about 0.9 mm. The use of PA 6 alone, or copolyamides thereof, did not lead to the desired success. PA 6 alone is on account of its low melting point (about 218.degree. C.) not usable at the contemplated vulcanization temperatures for heavy tires, for example heavy goods vehicle tires.